What is the definition of "unused site"?

Dear all,

I am interested to setup "Confirm site use and deletion" following this article:
http://technet.microsoft.com/en-us/library/cc262420(v=office.15).aspx

However I do not find the definition of "unused site". Is it defined by last modified date of any file within the site collection, or the last visit date of site collection?

In the same article it said "If the site collection owner confirms that the site collection is active by clicking the confirmation link in the email notification, thecertification date of the site collection is renewed." So there is an attribute "certification date" for each site collection? Where can I find it?

Thanks for any input.

Mark